Support QGIS server getLegendGraphics print-legend

[filleg]

Is your feature request related to a problem? Please describe.
The print-legend sends a tailored getLegendGraphics request for Geoserver. There is no handling for QGIS server that needs a different request. Instead of Legend.rules QGIS expects nodes.symbols and the json-response comes under title, not name. A few different parameters is required also such as WIDTH and HEIGHT.

Describe the solution you'd like
Different handling when the source is QGIS server.

Describe alternatives you've considered
Open for suggestions.

Additional context
Add any other context or screenshots about the feature request here.